PLANKTONIC FEEDING AND EVOLUTIONARY SIGNIFICANCE OF THE LOBATE BODY PLAN WITHIN THE CTENOPHORA

Ctenophores are gelationous marine invertebrates that can significantly affect zooplankton community structure by predation. The two main ctenophore orders, Cydippida and Lobata, have different feeding mechanisms: cydippids use two elongate tentacles to capture prey while lobates use large, flap-like lobes and numerous comparatively reduced tentacles suspended between the lobes. Cooccurrence of Bolinopsis infundibulum (Lobata) and Pleurobrachia pileus (Cydippida) in coastal areas where both species can reach high densities suggests that dietary overlap and feeding competition may occur. To examine this hypothesis, we recorded the gut contents of >150 individuals of each species collected simultaneously on six dates in spring 1998 at Woods Hole, Massachusetts. An emergent pattern of prey partitioning between the two species was evident. The cydippid P. pileus consistently consumed larger, more strongly swimming prey, such as gammarid amphipods, crab zoea, calanoid copepods and barnacle cyprid larvae. In contrast, the lobate B. infundibulum selected smaller, more weakly swimming prey, such as copepod nauplii, gastropod veligers, rotifers and tintinnids. Differences in prey selection reflected the mechanical bases of prey capture by each species. We propose that the capacity of lobates to exploit microzooplankton was a major selective force favoring the evolution of lobate ctenophores from cydippid ancestors.